Replace o-ring on Ideal Standard ‘Silver’ Bidet

Ideal Standard 'Silver' Bidet has this little swivelling nozzle on the spout. The o-ring on this deteriorated over time, leading first to leakage around the nozzle, then it came off all together. Photo shows the various parts:

• A: Remaining end of tap; note bulbous shape.
• B: Inner part of swivelly nozzle (bottom view).
• C: Inner part of swivelly nozzle (top view).
• D: Outer part of swivelly nozzle with worn-out o-ring.

I have a heap of replacement o-rings of different sizes, but I can't figure out how to put it back together. Clearly B/C screws together with D. I surmise (perhaps incorrectly) that the o-ring serves, not only to provide a water seal, but also to physically retain B/C on the bulbous part of A. But there's not much of a lip inside C to retain the o-ring. And how to put the pieces together? I can preinstall the o-ring over A, but can't then persuade it into C. I can preinstall in C, but when I try to push it on A, the o-ring just slides down inside A, refusing to ride over bulbous A.

Any ideas?